1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to method and apparatus for depositing semiconductor processing fluids on one or both sides of a semiconductor wafer.
2. Description of the Related Art
The fabrication of modern integrated circuits requires the patterning of millions of different types of regions on a semiconductor wafer, such as local interconnect trenches, global metallization layers, and transistor gates, to name just a few. The manufacturer of such multitudes of tiny structures is made possible by the use of lithographic processing. In photolithographic processing, various layers of photoresist materials are spin-coated onto the wafer. Next, the photoresist layer(s) is exposed to an actinic radiation source, normally ultraviolet. The UV radiation is first passed through a mask or reticle that selectively passes some of the UV radiation while blocking other portions so that only preselected portions of the photoresist are exposed to the radiation. The radiation changes the chemical character of the photoresist, either rendering it soluble or insoluble in a subsequent solvent step, depending upon whether the resist is negative photoresist or positive photoresist. The resist is then developed by exposure to a developer solvent. The areas of the photoresist remaining after the development step protect the substrate regions which they cover.
In conventional processing, the wafer is typically pre-baked and coated with a photoresist primer that promotes the adhesion of the photoresist to the underlying substrate material. The pre-baked step is carried out to remove water from the substrate that might otherwise interfere with the adhesion of the photoresist to the substrate. The primer is commonly applied by spin coating or vapor deposition. Hexamethyldisilazane ("HMDS") is an example of a photoresist primer, and is widely used in photoresist processing.
Following the cleaning, pre-bake, and primer steps, the wafer is coated with photoresist. Spin coating is a process frequently employed to apply photoresist to a substrate. In conventional processing, the spin coating procedure involves three stages: a) dispensing the resist solution onto the wafer; b) accelerating the wafer to a final rotational speed; and c) spinning at a constant speed to establish the desired thickness and to dry the film. The dispensing stage is accomplished by flooding the entire wafer with resist solution before commencing the spin cycle, or by dispensing a smaller volume of resist solution at the center of the wafer and spinning at relatively low speeds to produce a uniform liquid layer across the wafer. In some conventional processes, the wafer is held stationary while the resist is dispensed. In the acceleration stage, the wafer is quickly ramped up to the desired spin speed. High ramping rates generally yield better film uniformities than lower ramping rates. The constant speed spin cycle establishes a relatively uniform profile for the photoresist across the wafer and evaporates the remaining solvent to produce a solid film of photoresist.
There are several disadvantages associated with conventional photoresist deposition processing. Conventional apparatus for dispensing photoresist on a wafer require the wafer to be disposed in a horizontal or flat orientation. As a result, such systems are prone to unwanted dripping of photoresist onto the wafer from the dispensing nozzle which can lead to defects in the photoresist. To compensate for this problem, many conventional photoresist dispensing pumps now incorporate a suck-back action, which draws back excess photoresist from the dispensing nozzle into the supply line. The suck-back feature has not eliminated all dripping problems and has introduced design complexities associated with air bubbles becoming entrapped in the photoresist supply line which can lead to air bubbles becoming entrapped in the photoresist and causing defects. In addition, the horizontal orientation of wafers in conventional photoresist dispensing machines can lead to a phenomenon known as dishing. In dispensing machines where the wafer is secured to a spinning chuck by vacuum, excess vacuum force applied near the center of the wafer may cause the wafer to deform slightly and take on a dish-like profile. As a result, resist can puddle at the center of the wafer. Finally, for relatively large wafers, e.g., above 450 mm. in diameter, surface tension effects may lead to undesirable variations in the thickness of the photoresist film, particularly at the outer edges of the wafer.
Another disadvantage associated with conventional photoresist processing machines is the inability to efficiently coat both sides of a dual sided wafer. Conventional machines are configured to deposit photoresist on one side of a given wafer at a time. If coating of both sides of a dual sided wafer is contemplated, one side of the wafer must first be coated in the machine, and then the wafer must be lifted out of the spin bowl, flipped and repositioned in the spin bowl, and a second and separate coating step must be performed. This may be a time consuming operation and requires very delicate handling of the wafer to avoid damaging the coated side while photoresist is applied to the uncoated side.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.